Cross-linked polyethylene derived from low density polyethylene is widely used as a CV cable (cross-linked polyethylene insulated vinyl jacket power cable) insulative material because it is excellent in electric characteristics and heat resistance. However, it has a disadvantage in that dielectric breakdown occurs at superhigh voltages. It has therefore been desired to develop a material exhibiting more improved characteristics
Various attempts have been made to improve the dielectric breakdown characteristics at superhigh voltages, for example, as described in T. Fukada et al., I.E.E.E., E117 (5), (1982).
If there are impurities such as voids, water and metal, electric charges are concentrated to the impurities, thereby causing a reduction in the dielectric breakdown characteristics. For this reason, investigations have been directed mainly to the technique of removing such impurities. Thus, a clean polyethylene not containing impurities having a size of 250 .mu.m or more and a dry cross-linking method not producing voids have been developed. These techniques are described, for example, in M. Takaoka, I.E.E.E., Trans. Power Appar. Syst., 102 (9), 3254-3263 (1983). If, however, a 275 kv cable is produced by these conventional methods, the thickness of the cable insulative material reaches several centimeters. It is, therefore, necessary for the cable insulative material to have more improved dielectric breakdown characteristics.
Another attempt is to add a voltage stabilizer (e.g., calcium stearate, polystyrene and aromatic compounds) to the cable insulative material so as to provide it with a capability to prevent the concentration of electric charges, as described, for example, in U.S. Pat. No. 3,346,500 and 3,350,312. The reason for this is that if the cable insulative material possesses a capability to prevent the concentration of electric charges by itself, the thickness of the cable insulative material can be decreased. The addition of such an additive, however, suffers from a disadvantage that the additive bleeds out and the performance of the cable cannot be maintained over a long period of time.
In recent years investigations have been extended to other materials such as high density polyethylene, polypropylene and polystyrene, as described, for example, in K. Kaminaga et al., Proceeding of the 17th Symp. on Electrical Insulating Materials, p-7, 193 (1984) Japan. Since, however, the cable is coil-like wound, it is required for the cable insulative material to have flexibility. If, therefore, the above materials are used, it is necessary to provide flexibility to them. In resins produced by coordinate anionic polymerization, such as high density polyethylene and polypropylene, catalyst residues such as Ti and Al remain unremoved although their amounts are small, and there is a danger that they become sites for the concentration of electric charges.